Free Plug It In Reminder App for BlackBerry Devices.

With Plug It In simply set at what level you want your device to notify you when your battery drops below that point. A pop-up warning will alert you. You can also configure the LED to flash and sound to play.

Once your battery drops below your set level a new notification icon will appear on your screen showing you how much battery life you have left. After you charge your phone again past that level it will disappear. (This feature only works on OS 4.6 and above)